1. Field of Invention
This invention relates generally to electromagnetic flowmeters, and more particularly to a flangeless flowmeter having a cylindrical housing and whose components are integrated to form a highly compact, low-cost unit that may be readily installed in a flow line between the flanged ends of the upstream and downstream pipes, the flowmeter including pre-cast electrodes of conductive epoxy material which are embedded in and bonded to a molded pressure vessel of insulating epoxy material to create a monolilithic structure.
2. Prior Art
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,738, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
The typical commercially-available magnetic flowmeter is provided with mounting flanges at either end thereof. The meter is interposed between the upstream and downstream pipes of a fluid line, each pipe having an end flange. The mounting flanges on the meter are bolted to the flanges of line pipes. It is, of course, essential that the circle of bolt holes on the mounting flanges of the meter match those on the pipe flanges.
In a magnetic flowmeter, the flow tube is subjected to the same fluid pressure as the line pipes. The flow tube must therefore be of a material and of a thickness sufficient to withstand this pressure, even though the strength of the flow tube is unrelated to its measuring function. This design factor contributes significantly to the cost of a standard meter. Existing meters are made up of components that must be assembled, and are generally of substantial size and weight and quite expensive to manufacture.
In order to provide a compact and readily installable electromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, the above-identified related patent applications and patents disclose highly compact flangeless flowmeters which, despite their reduced volume and weight, are capable of withstanding high fluid pressures, the flowmeters operating efficiently and reliably to accurately measure flow rates.
In applicant's related U.S. Pat. No. 4,098,118, the flowmeter is constituted by a cylindrical metal casing within which a pair of solenoids are supported at diametrically-opposed positions along a magnetic axis at right angles to the longitudinal axis of the casing which passes through the central flow passage of an annular pressure vessel.
This vessel, which is formed of high-strength insulating material, is molded within the casing and encapsulates both the solenoids and a pair of metal electrodes disposed at diametrically-opposed positions with respect to the flow passage along a transverse axis at right angles to the magnetic axis to define a unitary structure. This flangeless unit is compressible between the end flanges of the upstream and downstream pipes of the fluid line by bridging bolts which pass through bore holes in the pressure vessel or which lie outside the casing to encage the unit.
The faces of the electrodes are in contact with the fluid. While the electrodes are embedded in the pressure vessel and locked therein, the metallic surfaces thereof are not chemically bonded to the insulating material of the pressure vessel, nor does the metal of the electrodes have the same coefficient of thermal expansion as the insulating material.
As a consequence, the seal at the interface of the metal electrodes and the insulating vessel is weak and subject to rupture under high pressure or high-temperature operating conditions. Because the electrodes are embedded, a seal failure is not repairable; hence should the seal fail, it may be necessary to discard the entire unit.
In other metal electrode arrangements disclosed in the prior art, the meter electrodes are received in holders, gaskets and compression seals being provided to maintain a good seal under rigorous operating conditions. But such expedients add substantially to manufacturing and assembly costs.
Another drawback of metal electrodes is that when the fluid being metered is in the form of a slurry in which solid particles are dispersed in the fluid, the impingement of these particles on the metal faces of the electrodes give rise to noise which in some cases is so intense as to result in an unfavorable signal-to-noise ratio.